Generally speaking, such sensors are composed of an interrogation unit (itself composed of a transmitting part and a receiving part) and of a temperature sensor using surface acoustic waves, commonly denoted by the acronym SAW. The interrogation system together with the SAW sensor are equipped with an antenna adapted to the operating frequency band (ISM band 433 MHz, 868 MHz, 2.45 GHz, etc.) which allows a wireless interrogation of the sensor to be carried out. FIG. 1 shows schematically such a type of remotely interrogatable sensor. An interrogation/reception unit 10, equipped with an antenna 11, generates an interrogation signal SERF in the direction of a SAW sensor 20 also equipped with an antenna 21 (low-frequency time-domain pulse of a carrier within the ISM band). The SAW device can advantageously be of the resonator type which allows access to structures of reduced sized.
If the transmission signal SERF has a frequency sufficiently close to the natural resonance frequency of the SAW resonator, the latter goes into resonance after going through a charging period. A permanent oscillation regime SOREF is then established at the natural resonance frequency of the SAW device. This resonance frequency is proportional to the speed of the surface wave in the resonant cavity which itself depends on the temperature of the resonator.
The sensor re-emits a signal SCREF at its resonance frequency which carries the information associated with the quantity to be measured, for example the temperature.
The transmission/reception unit for the interrogation system detects, outside of the transmission time frame, all or a part of the SAW signal (damped oscillation) and extracts from it the information sought, for example the temperature, via an suitable processing of the signal.
Typically, the resonator can be composed of an interdigitated comb transducer, composed of an alternation of electrodes with widths that are repeated with a certain periodicity, known as acoustic period, deposited onto a piezoelectric substrate which can advantageously be made of quartz. The electrodes, advantageously made of aluminium (formed by a photolithographic process), have a thickness that is small compared with the acoustic period (typically, a few hundreds of nanometres to a few micrometres). For example, for a sensor operating at 433 MHz, the thickness of metal (aluminium) used can be of the order of 1000 Angstroms, where the acoustic period and the electrode width can respectively be around 3.5 μm and 2.5 μm.
One of the ports of the transducer is for example connected to a radio frequency (RF) antenna and the other to ground. The field lines thus created between two electrodes with different polarities give rise to surface acoustic waves in the overlapping region of the electrodes.
The transducer is a bi-directional structure, in other words the energy radiated towards the right and the energy radiated towards the left have the same intensity. By disposing electrodes on either side of the transducer, the former acting as a reflector, a resonator is formed, each reflector partially reflecting the energy transmitted by the transducer.
If the number of reflectors is multiplied, a resonant cavity characterized by a certain resonance frequency is created. This frequency principally depends on the speed of propagation of the waves under the array, the latter mainly depending on the physical state of the substrate, and hence for example sensitive to its temperature. In this case, this is the parameter which is measured by the interrogation system and it is using this measurement that a temperature can be calculated.
It is recalled that the variation of the resonance frequency of a resonator on quartz is determined by the following formula:f(T)=f0[1+CTF1(T−T0)+CTF2(T−T0)2]  (1)with f0 the frequency at T0, T0 the reference temperature (25° C. by convention), CTF1 the first order frequency temperature coefficient (ppm/° C.) and CTF2 the second order frequency temperature coefficient (ppb/° C.2).
This law may also be refomulated bringing in a temperature for inversion of the law (1), referred to as turn-over temperature:f(T)=fTt+f0CTF2(T−Tturn-over)2  (2)with fTt the frequency at the turn-over temperature and Tturn-over the turn-over temperature;
These quantities are given by the following equations:Tturn-over=T0−CTF1/2CTF2 fTt=f0[1−CTF12/4CTF2]  (3)
The law of variation of the resonance frequency as a function of temperature is therefore a parabola; the temperature at which the frequency is maximum (summit of the parabola) is known as the turn-over temperature.
It can be particularly advantageous to use two SAW resonators (W. Buff et al., “Universal pressure and temperature SAW sensor for wireless applications” 1997 IEEE Ultra. Symp. Proc.) inclined with respect to one another, as illustrated in FIG. 1. In this case, a first resonator R1, for which the direction of propagation of the surface waves is in a direction X corresponding to one of the crystallographic axes of the crystalline substrate, is coupled to a second resonator R2, inclined by a certain angle α (which can typically be around 20°) with respect to the axis X, and hence using another direction of propagation.
The advantage of such passive temperature sensors resides in the fact that they can be interrogated remotely and hence that it is possible to locate the interrogation and processing unit outside of the heating chamber of the type oven, autoclave, etc., in which the passive sensor is placed, only the transmission/reception antenna, equivalent to the antenna 11 illustrated in FIG. 1, being placed in the said chamber.
Nevertheless, the RF signal becomes a source of multiple reflections of energy within the metal chamber, generating a spatial distribution of the RF energy which is dependent on the size and on the shape of the chamber in which the passive sensor is placed, and which is also dependent on elements seen as obstacles which could also be placed inside of the said chamber.
The spatial distribution of energy then exhibits minima and maxima of power values. If the sensor is positioned near to a minimum value, it may become impossible to interrogate the said sensor remotely. It will nevertheless be sought to optimize the energy efficiency and to optimize the interrogation method.
It may be envisaged to precisely position both the transmission antenna and the sensor within the chamber in order to adopt an optimal configuration taking into account the aforementioned parameters and notably the size and of the shape of the cavity.
For each configuration, it is however necessary to carry out an energy assessment and an appropriate positioning of the said sensor, this problem becoming even more acute in the presence of several sensors.